Placement of the port is only part of the equation when designing a vented enclosure. In fact, it is the least important part.
First of all, if you plan to put both subs in your car, they should be the same brand/model and installed in the same type/size enclosure. Mix-matching equipment rarely does you any good. In most cases it just causes more cancelation and less bass.
Second, in order to successfully increase the amount of bass from a subwoofer by porting, you have to design the box based around the TS parameters of the subwoofer. Ported boxes, as a rule of thumb, are larger than sealed boxes so you can't use your existing box. You will have to design a new one from the ground up.
The first place to look for a design is the manufacturer's instruction manual or website. If they provide a recommended ported enclosure design you should go with that. If they don't, you'll have to use the TS parameters and a box design program to come up with the optimum design for your subwoofer. The size, shape, port diameter, and port length all play an important role.
If I have confused you in any way...chances are you are not up for the task. Just throw the subs in the trunk and leave it at that.